1. Field of the Invention
The present invention relates to a connection-switched capacitor storage system comprising a plurality of capacitors, monitors connected in parallel with the capacitors, respectively, switches for switching the connections of the capacitors from a series combination to a parallel combination or vice versa, and control means. Each parallel monitor acts to bypass the charging current when the terminal voltage of the corresponding capacitor exceeds a given voltage value. Thus, the parallel monitors limit the terminal voltages of the capacitors to a voltage set for initializing. That is, the parallel monitors have a function of initializing the capacitors to their original state (hereinafter often referred to simply as initialization or initializing state). The control device controls the initializing operation of the parallel monitors according to the terminal voltages of the capacitors. The control device also controls the switching operation of the switches for switching the connections of the capacitors.
2. Description of Related Art
A capacitor storage system consisting of a combination of capacitors and an electronic circuit is known as an ECS (energy capacitor system). Those energy capacitor systems which are equipped with parallel monitors having a function of initializing capacitor voltages to their initial level and which have a function of switching the connections of the capacitors have been studied and verified in terms of their performance in a Japanese national project NEDO (New Energy and Industrial Technology Development Organization): Final Report of on-the-spot Research on new Procedure for Load Leveling, March 2000. Its performance has been valued highly and put into practical use.
Electric storage systems equipped with parallel monitors having a function of initializing capacitor voltages to their initial level have been proposed by the present Applicants and others, for example, in Japanese Patents Laid-Open Nos. 2000-152508, 2000-217250, and 2001-186681 (U.S. Pat. No. 6,404,170).
Electric storage systems having a function of switching the connections of capacitors have been also proposed by the present Applicants and others, for example, in Japanese Patents Laid-Open Nos. 2000-152495 (U.S. Pat. No. 6,133,710), 2000-209775, and 2000-253572 (U.S. Pat. No. 6,317,343).
An example of the structure of an electric storage system fitted with parallel monitors having a function of initializing capacitor voltages to their initial level is now given. FIG. 5 shows one example of the configuration of a capacitor storage portion having comparators acting as parallel monitors which are used, respectively, for initializing and for detection of a full charge condition. Shown in this figure are a charger 11, comparators 12, 13, OR-gates 14, 15, capacitors C, diodes D, resistors Rs, transistors Tr, and initializing switches S1. Vful and Vini indicate set voltages, respectively.
In FIG. 5, each capacitor C is an electric double-layer capacitor, for example, for storing electrical energy. The comparator 12 for initializing to initial state is used as a means for operating the transistor Tr connected in parallel with the capacitor C in such a way that the charging current is bypassed at the first set voltage Vini. The comparator 13 for detection of a full charge is used as a means for detecting the second set voltage Vful to judge that a full charge voltage higher than the first set voltage has been reached. When the capacitor C is initialized to its original state, if the terminal voltage of the capacitor C is about to exceed the set voltage Vini, the transistor Tr and resistor Rs together form a bypass circuit for the charging current, thus limiting the charging current. That is, a part of the charging current is bypassed. The current is set by the resistor Rs. The initializing switch S1 activates or deactivates the operation for initializing the capacitor C. When the initializing mode is selected, an initialization execution signal S issued by the charger 11 activates the operation.
The charger 11 charges plural capacitors C connected in series. The charger 11 stops the charging operation if a full charge voltage is detected from any capacitor C. For example, the outputs F from the comparators 13 for detection of a fully charged state are logically ORed. Thus, the charger judges which of the plural capacitors has reached full charge. Then, the charging is ended. Furthermore, when charging for initialization is started, the charger 11 turns on (closes) the initializing switch S1 by the initialization execution signal S, thus starting charging. The outputs I from the comparators 12 for initializing the capacitors are logically ORed. Thus, the charger judges which of the capacitors has started to undergo an operation for bypassing the charging current. The bypass operation signals I from the comparators 12 are ORed by each OR gate 14. Output signals F from the comparators 13 indicating a full charge are ORed by each OR gate 15, and a signal for stopping constant-current charging is supplied to the charger 11.
Accordingly, the set voltage Vful is set to the full charge voltage of each capacitor. The set voltage Vini is set to an initializing voltage lower than the set voltage Vful. When the initializing switch S1 is closed (turned ON) and charging is done, the capacitor charged to the set voltage Vini first is first started to be charged at a decreased charging rate by the bypass circuit consisting of the transistor Tr and resistor Rs by bypassing a part of the charging current. In this way, the capacitors are successively charged at a decreased charging rate. When any capacitor reaches full charge, the charger 11 stops the constant-current charging. If necessary, trickle charging is done.
An example of the configuration of an electric storage system having a function of switching the connections of capacitors is next described. FIGS. 6a to 6c show one example of the configuration of a capacitor storage system in which the connections of capacitors are switched. Shown in these figures are capacitors CA1-CA3, CB1-CB3 and switches SS, SA1-SA3, SB1-SB3.
Referring to FIGS. 6a to 6c, the capacitors CA1-CA3 and CB1-CB3 form two sets of capacitors A and B. Each set of capacitors is made up of the same number of capacitors connected in series. Each of the capacitors CA1-CA3 and CB1-CB3 may be a capacitor bank consisting of plural capacitors connected in series or parallel-series. If necessary, a parallel monitor is appropriately connected with each capacitor. The switch SS is a series-connection switch for connecting the two sets of capacitors A and B in series. One set of capacitors A and the switch SS are connected at a series connection point. The switches SA1-SA3 are one set of switching means for connecting this series connection point with one series connection point of the other set of capacitors B and with the series connection points between the capacitors CB1-CB3. The switches SB1-SB3 are the other set of switching means for connecting the series connection point between the set of capacitors B and the switch SS with the other series connection end of the set of capacitors A and with the series connection points between the capacitors A.
Then, the capacitors CA1-CA3 and CB1-CB3 are connected in series as shown in FIG. 6d by closing only the switch SS as shown in FIG. 6a. The center capacitor CA3 of one set of capacitors A and the center capacitor CB3 of the other set of capacitors B are connected in parallel as shown in FIG. 6e by opening the switch SS and closing the switch SA3 of one set of switching means and the corresponding switch SB3 of the other set of switching means as shown in FIG. 6b. 
Similarly, the series combination of the center capacitors CA3 and CA2 of one set of capacitors A and the series combination of the center capacitors CB3 and CB2 of the other set of capacitors B are connected in parallel as shown in FIG. 6f by closing the switch SA2 of one set of switching means and the corresponding switch SB2 of the other set of switching means and opening all the other switches as shown in FIG. 6c. 
Then, the series combination of the capacitors CA1-CA3 of one set of capacitors A and the series combination of the capacitors CB1-CB3 of the other set of capacitors B are connected in parallel as shown in FIG. 6g by closing the switch SA1 of one set of switching means and the corresponding switch SB1 of the other set of switching means and opening all the other switches.
As described above, the connections of the plural capacitors CA1-CA3 and CB1-CB3 are switched and controlled as shown in FIGS. 6d to 6g, by selectively connecting one of the switches SA1-SA3 of one set of switching means and one of the switches SB1-SB3 of the other set of switches or the switch SS. In this way, the voltages are adjusted. Variations in the voltages accompanying charging and discharging can be suppressed. For example, the capacitors CA1-CA3 and CB1-CB3 are all connected in series and charging is started as shown in FIG. 6d. When the terminal voltage on the charging side rises to a given value, the voltage is lowered by an amount corresponding to the capacitors CA3 and CB3 by switching the combination to the combination shown in FIG. 6e. Furthermore, if the terminal voltage on the charging side again increases to the given value due to charging, the terminal voltage on the charging side can be prevented from exceeding the given value by switching the combination successively to the combinations respectively shown in FIGS. 6f and 6g. 
Where discharging is started in the connection combination shown in FIG. 6g and the load is fed, if the output voltage drops to the given value, the decrease in the output voltage is compensated by switching the connection combination to the combination shown in FIG. 6f. If the output voltage further drops to a certain value, the connection combination is successively switched to the connection combinations respectively shown in FIGS. 6e and 6d. Consequently, the output voltage can be prevented from decreasing below the certain value. Furthermore, the overall current flowing during charging and discharging is allocated to only the switch SS that connects all the capacitors CA1-CA3, CB1-CB3 in series. The other switches SA1-SA3 and SB1-SB3 only need to have a current capacity that is half of the overall current. In addition, only one switch is connected in series with each capacitor at any stage. Therefore, loss caused by turn-on voltage of switches, which would present a problem where the switches were made of semiconductors, can be reduced to a minimum.
In the system used thus far, however, parallel monitors having a function of initializing capacitors to their initial state and a function of switching the connections of the capacitors between series and parallel combinations are combined in a simple manner as mentioned previously. Therefore, there arises the case where both functions perform conflicting operations. It has been confirmed that the energy efficiency of the capacitor storage system can drop.
The observed decreases of the efficiency are only 1% to 2%. However, the actual value of the overall charge/discharge efficiency of the whole capacitor storage system using switching of the connections of capacitors is as high as 94%. Therefore, where the decreases of the efficiency are only 1% to 2% as mentioned previously, increasing the efficiency further will greatly contribute to expansion of the application of the capacitor storage system.